1. Field of the Invention
The present invention relates to a method and apparatus for controlling the ignition time or angle for every rotation cycle of the output shaft of an internal combustion engine in dependence on the operating conditions thereof. More particularly, the invention concerns the ignition timing control of the above type where means is provided to prevent the ignition times in any two successive rotation cycles from differing remarkably from each other.
2. Description of the Prior Art
Recently, strict requirements have been imposed on the internal combustion engines for motor vehicles in respect of reduction in consumption of fuel and purification of the exhaust gases. In order to meet such requirements, the ignition timing of the engine has to be controlled with high accuracy in dependence on the operating state of the engine. For example, a parameter representing an operating condition of the engine such as the number of rotations is finely divided into a number of gradations thereby to control the ignition timing as a function of such a finely graded parameter. Certainly, the above requirement may be satisfied by such fine ignition timing control. However, there arises the possibility that the ignition timing undergoes significant variation in response to even a slight change in the operation state of the engine, bringing about correspondingly large variation in the output torque of the engine and instability in the engine operation.
In general, it is considered most preferable that the ignition be effected with a minimum advance angle at the maximum torque (i.e. minimum advance for best torque or MBT) in view of the operation and fuel consumption efficiency of the engine. However, when the ignition timing is advanced excessively, there may occur a so-called knocking phenomenon. In order to assure stable operation, the ignition timing has to be adjusted so that such knocking phenomenon is suppressed.
From the viewpoint of a high purification ratio of the exhaust gas, the ignition timing of the engine has to be controlled finely, which in turn involves a steep ignition timing control characteristic, i.e. the ignition timing control is very sensitive to variation in the operating condition of the engine. With such a steep characteristic of the ignition timing control, even a slight transient variation in the operating state of the engine will give rise to a significant hunting phenomenon in addition to a remarkable degradation in the operation stability of the engine.